Deep waters provide nutrients that plankton need for primary production in the upper ocean, but how do these nutrients get to the ocean depths? They are carried down from the surface in a rain of particles often referred to as marine snow, which includes fecal pellets from zooplankton, shells from dead plankton, and other bits of organic material from dead or dying microorganisms. Many of these bits are almost invisible and too light to sink (a fecal pellet is about the size of a typeset comma), but when they stick together and form clumps called aggregates, the clumps eventually become heavier. Bacteria, plankton, and larger creatures feed on the pellets as they sink, then excrete wastes that add to the snowfall. Particles can travel to the deep ocean floor in spans of days or weeks.

When marine snow reaches deep waters, some is consumed by bottom-dwellers and microbes who depend on it as a food source since they live below the zones where light penetrates. Some is oxidized, releasing CO2, nitrate, and phosphate and recycling nutrients into deep waters. The remainder is buried in sediments; organic-rich carbon buried thousands of years ago in ancient inland seas and coastal margins is the source of today's offshore oil and gas deposits.

In addition to recycling nutrients and providing food for deep-dwelling species, this flow of particles to ocean depths is a critical link in the global carbon cycle, dubbed the biological pump by oceanographer and climate scientist Roger Revelle. Plankton take up carbon from the atmosphere in two ways: they fix CO2 as organic carbon during photosynthesis and form shells from calcium carbonate. Marine snow carries both of these forms of carbon away from the atmosphere and surface waters to reservoirs in the deep oceans and ocean sediments, where it remains stored for centuries (Fig. 14).

Without this mechanism, concentrations of CO2in the atmosphere would be substantially higher—and since atmospheric CO2 traps heat, Earth's surface temperature would be significantly higher. (For more details on the greenhouse effect, see Unit 2, "Atmosphere.") "If there were no phytoplankton—if the biological pump did not exist and the oceans all mixed from top to bottom and all that CO2 in the deep oceans equilibrated with the atmosphere—the concentration of CO2 in the atmosphere would more than double," says MIT's Chisholm. "Phytoplankton keep that pump pumping downward."

The overall efficiency of the biological pump depends on a combination of physical and biogeochemical factors. Both light and nutrients must be available in sufficient quantities for plankton to package more energy than they consume. Enough particles must sink to recycle nutrients into deep waters, and upwelling must occur to bring nutrients back to the surface. Factors that can impede this process include ocean warming (which makes the sea's layers more stratified, preventing mixing that brings up nutrients), and pollution and turbulence, which can reduce the penetration of sunlight at the surface.